1. Field of the Invention
The invention relates to connecting and disconnecting a nuclear reactor control element assembly with its drive extension shaft through actuation of a spring which is part of the buffer mechanism. More particularly, the invention relates to coupling structure between a drive extension shaft and nuclear reactor control element assembly which is locked by the spring of the buffer mechanism.
2. Description of the Prior Art
In a nuclear reactor, control elements are inserted into the core, of fuel region, of the reactor to control and regulate its reactivity and power level. These control elements contain materials known as poisons which absorb neutrons and lower the local neutron flux. In normal operation, the control elements are withdrawn, at least partially, from the core region, and their position may be controlled to regulate the reactor. In the event of an emergency in which the reactor must be shut down, it is necessary to rapidly insert most, or all, of these control elements fully into the core. This emergency procedure of inserting control elements is termed "scramming".
Control elements which enter the reactor core from above, and are withdrawn from the core to a position thereabove, can be scrammed by allowing the control elements to fall into the core by the forces of gravity. This is normally accomplished by simply disengaging the control elements and drive train from the drive mechanism. It is necessary, however, to provide means for gradually slowing down or decelerating the control elements just prior to their reaching their extreme lower position in the core to prevent damage to the reactor structure or to the control elements. These decelerating structures are termed buffer mechanisms.
Various buffer mechanisms associated with individual control elements or with a control element assembly, including a plurality of control elements, have been provided to effect the controlled deceleration of the control elements. One type of buffer mechanism often used is that of a piston and cylinder combination. The piston, or cylinder, is fixed to the core structure and the cylinder, or piston, is carried by the control element assembly. Fluid in the reactor is in the cylinder when the control element assembly is suspended above the reactor core. When the assembly is allowed to fall (scram), the piston-cylinder engages and the fluid in the cylinder acts upwardly against the falling control element to effect a gradual deceleration of the control assembly. In addition to this fluid force, a spring is positioned in the cylinder to exert its force against the end of the piston and the structure on which the cylinder is mounted.
System 80 nuclear reactors use a particular form of piston-cylinder buffer mechanism between their control element assemblies and the core structure. This form is disclosed in United States patent application Ser. No. 546,410 filed Feb. 3, 1975 for the invention entitled "Shock Buffer for Nuclear Control Element Assembly" and the disclosure of that application is hereby incorporated by reference. In general, the spider of the control element assembly descends on a buffer pin which fits, as a piston, into a dashpot cylinder of the spider.
The normal positioning of the control element assembly is with a drive mechanism on the exterior of the reactor vessel. From the drive mechanism a drive extension shaft extends through the reactor vessel head to connect with the spider. Specifically, on the upper side of the spider is a ball. On the end of the extension shaft is a releasable gripper, or collet fingers, which capture the spider ball. These collet fingers, as a releasable gripper, are normally disengaged from the spider ball preparatory to refueling.
The present coupling system requires an operating rod to actuate the gripper of each drive extension shaft. As there are up to ninety-seven control rod drive mechanisms for each System 80 reactor, there is a significant saving potential in providing a single operating rod to release the grippers of all drive extension shafts. At the same time, if the buffer mechanism is rearranged to apply a single spring force to the gripper, in locking the extension shaft to the control rod spider, and the buffering, further savings and simplification can be realized.